The PC5 interface facilitates a direct link (on a so-called ‘sidelink’) between UEs (User Equipments). A PC5 resource pool may be used by a P-UE (Pedestrian User Equipment) and shared with V-UEs (Vehicle User Equipment) or I-UE (Infrastructure User Equipment). Herein, the term V2X means LTE-based (Long Term Evolution) communications services connecting vehicles, pedestrians and infrastructures:P2V refers to LTE-based communication between a device carried by an individual, such as a pedestrian, and a vehicle. V2V refers to LTE-based communication between vehicles: V2P refers to LTE-based communication between a vehicle and a device carried by an individual: V2I refers to LTE-based communication between a vehicle and a roadside unit. A roadside unit may be implemented in an eNB or static UE (User Equipment).
One current proposal under consideration for P2V transmissions is ‘random resource selection.’ This minimizes power consumption for PC5-based V2X services and does not require the P-UE to receive on a PC5 carrier. Also, random resource selection is able to meet the requirement of V2X services including the packet reception ratio (PRR) performance requirement. A proposal under consideration for V2X resource selection is a sensing operation which is performed during a limited time. This involves energy measurements or SA (Scheduling Assignment) decoding. Due to better selection of resources and reduced resource collisions (compared with random resource selection), this can improve the PRR performance although with the penalty of additional power consumption and the need for UE reception on a PC5 carrier.
The sensing operation is performed in order to determine the available resources to be used. The sensing operation is currently defined for UEs which are mounted in vehicles and connected to (almost) unlimited power supply. The existing traffic model of V2X is re-used; one 300-byte message is followed by four 109-byte messages and transmission periodicities are k*100 ms where k=1, 2, 3, 4, 5, 6, 7, 8, 9, 10. The periodicity is determined by the speed of the vehicle in question; For example, a higher speed vehicle may transmit every 200 ms. A lower speed vehicle may transmit every 800 ms. For P2X, the message size is fixed at 300 bytes and the transmission periodicity is 1 s.
During a known sensing operation, a V-UE monitors (i.e. at least decodes) transmitted data from other UEs and derives from the content if and when each UE will use the same resource for future transmissions. That is to say that the UE reserves these resources for future use. This information is necessary so that the sensing operation can mitigate the resource collision problem when resources are selected autonomously. Collisions can occur because UEs select resources from a pre-defined pool of limited resource. A resource reservation is related to the transmission periodicity (of the V2X transmissions). For example; if a V2X UE is currently transmitting every 200 ms, then the V2X UE will reserve a resource for its next transmission 200 ms ahead. A current specification deems that V-UEs will perform sensing during a ‘sensing window’ of 1 s duration. In this case, sensing can cover all periodicities and therefore V-UEs can ultimately avoid of collisions when resources are selected or re-selected.
For V2P operations, a P-UE consumes considerable power because it needs to receive messages which may be generated as frequently as every 100 ms by vehicles in its proximity. On the other hand, a P2V transmission has the potential to consume less battery power because a P-UE needs to access the channel only once in every 1000 ms. However, the exact amount of battery consumption is dependent of the P-UE sensing behaviour: If a P-UE is required to sense the channel to identify which resources are occupied by the other UEs, P-UE power consumption will increase proportionally to the time duration of this sensing. It has been proposed that a P-UE performs a sensing operation during a limited time, i.e. less than one second. This would potentially reduce collisions compared with a random resource selection but take more battery power.
One known proposal is for sensing to be done on a periodic window of less than 100 ms (e.g. 10 ms) every 100 ms (i.e. 10 windows of 10 ms are equally spread over 1 second). In this way, a P-UE can perform sensing for a reduced time but, since it is done with 100 ms periodicity, it can capture all transmissions with all periodicities whose future transmissions could fall into the P-UE's target window for transmission. This method is attractive in that the sensing covers all transmissions with all periodicities. However, the power consumption is still relatively high.
It would be advantageous to provide a means for minimising the collision probability of V2P and P2V communications without a P-UE having to perform a comparatively long sensing operation that degrades the battery life.